COMPATIBILITY BULLETIN Fault Tolerance Subsystems: Unitrol Immunity IMMUNITY 01.31.92 Compatibility Statement Unitrol's Immunity Disk Mirroring Software v1.x is compatible with LANtastic NOS version 4.xx running with MS-DOS 3.3 or higher. Product Overview Disk Mirroring, a process of operating 2 hard disk drives as one, can be achieved on a LANtastic network through the use of Unitrol's Immunity system. Any activity that occurs on one disk (the primary drive) is duplicated or "mirrored" on the second disk (the secondary drive). In case of hard disk failure on one of the drives, processing will automatically be continued on the remaining error-free disk, thus avoiding data loss and interrupts in system operation. Disk mirroring can also be used as a means of providing on-line backup. Product Installation Notes To start with, you'll need 2 physical hard disks within one PC. They may be of different capacity, brand and type, and can be connected to one or two disk controllers. The two disks must reside within the same PC as mirroring cannot be performed over the network. Immunity disk mirroring can reside on servers or workstations or both. Before installing the software, both drives must be partitioned using the FDISK command (from MS-PC/DOS 3.3 or higher) and formatted using FORMAT (also a DOS utility). Run INSTALL from the floppy disk that comes with Immunity, which will modify your config.sys and install the device driver IMMUNITY.SYS. It will also add 1 line in your autoexec.bat: IMMUNITY -b to verify that contents of the mirrored drives are synchronized every time you start your computer. INSTALL also performs a "Resynchronize Primary Disk To Secondary Disk", which mirrors all the selected data on the primary disk to the secondary disk. Upon completion of the "Resynchronization", both disks will contain identical data. From this point on, disk mirroring will be transparent to the operating system. Product Execution Notes Under normal operation, disk mirroring requires no attention. It sits invisibly in the background duplicating all writes to the secondary disk and monitoring the status of both disks. Just read the manual in advance to make sure that you follow the proper operation procedures in the event of a hard disk failure. When a disk failure does occur, a message is flashed on the top of the PC's screen indicating that a disk has failed. Operation then continues on the error-free disk, but you will no longer have disk-mirroring protection. It is strongly recommended that you repair or replace the defective disk, re-synch the data and then put disk-mirroring back on again. If drive c:(primary) is mirrored to drive d:(secondary), and the user attempts to access drive d:(secondary), s/he will get a general failure message. Performance Evaluations Since Immunity writes to 2 disks instead of 1, a small performance tradeoff should be expected. However, the system will try to compensate this by saving time during the read operations through a method known as Fastseek. Fastseek allows the system to read from the hard disk whose head is closest to the data. This also decreases the wear and tear on a hard disk due to head movement. Unitrol's lab tests have shown a 5-8% overall decrease in performance, since most systems do much more reading (about 80%) than writing (about 20%). Performance testing results of course depend on the benchmark tests being performed. Vendor Information Unitrol Data Protection Systems, Inc. 604-815 Hornby Street, Vancouver BC V6Z 2E6 Canada. (Head Office) 1480 Gulf Road, Point Roberts WA 98281 (US Office) Phone (604)681-3611 Fax (604)687-0814 Tech. Support (604)681-3613 U.S. Sales (800)665-2212 ARTISOFT, Inc. makes no warranties as to the completeness or accuracy of this document. LANtastic is a trademark of ARTISOFT, Inc. Brand names, company names, and product names are trademarks or registered trademarks of their respective companies.